The present invention relates to a magnetic bearing controller for holding a floating member such as the rotating shaft of a pump in a non-contact floating state using the magnetic attraction force of an electromagnet, and particularly to a technique for connecting the magnetic bearing to a sensor drive circuit on the controller end for detecting the floating position of the floating member. The present invention also applies to a turbo-molecular pump apparatus equipped with a magnetic bearing.
Magnetic bearings are used to support rotating shafts of pumps and the like in a non-contact state. Rotating shafts in a turbo-molecular pump supported by magnetic bearings, for example, are capable of rotating at a high rate of speed without causing wear and tear on the bearing. The magnetic bearing requires no lubricating oil and is therefore maintenance-free. These magnetic bearings include an active radial bearing for actively controlling the position of the rotating shaft in the radial direction or an active axial bearing for actively controlling the position of the rotating shaft in the axial (thrust) direction. A floating position controller in these magnetic bearings includes an electromagnet for supporting the floating member by the influence of magnetic attraction force, inductance-type displacement sensors for detecting the position of the floating member through variations in inductance, and a controller that controls the exciting current (i.e. magnetic attraction force) of the magnet based on the signal from the displacement sensors in order to support the floating member in the desired position.
In a magnetic bearing controller with this configuration, the electromagnet supporting the rotating shaft or the like and the sensor elements for detecting the position of the rotating shaft are integrally installed in a pump apparatus. However, the controller is ordinarily provided separate from the pump and connected to the pump via a cable. In this configuration the controller includes a power source for providing an exciting current to the electromagnet, a sensor circuit for receiving a signal detected by a sensor element and calculating the floating position, a PID (proportional plus integral plus derivative) control circuit for generating control output based on a comparison between output from the sensor circuit and the target floating position, and the like. Output from the sensor element installed in the pump is transferred to the sensor circuit in the controller via the cable.
FIG. 1 shows the construction of the sensor drive circuit in a conventional magnetic bearing controller. As described above, the magnetic bearing controller includes an electromagnet (not shown) supporting the floating member by magnetic attraction force; sensor elements Z1 and Z2, such as inductance-type position sensors, disposed near the electromagnet; and a control circuit that receives the output signals from the sensor elements Z1 and Z2 and supplies an exciting current to the electromagnet in order to support the floating member in a desired floating position. Also as described above, the magnetic bearing and controller are connected via a cable C.
A sensor drive circuit includes an AC signal source 11, an amplifier 12 for amplifying the signal from the AC signal source 11, a current limiting resistance 13, a resonance condenser 14, and the like. A circuit for measuring displacement in the floating member is configured by a bridge circuit bridging a central point between reference resistances Ra and Rb having a central point A connected to the output terminal of the control sensor drive circuit and the sensor elements Z1 and Z2 on the magnetic bearing side connected to the reference resistances via the cable and having a center point B. A differential amplifier extracts voltages from center points A and B between the serially connected Ra and Rb and Z1 and Z2, amplifies the signals, and outputs a displacement signal.
With this configuration, the bridge circuit linking the resistances Ra and Rb and sensor elements Z1 and Z2 calculates the position displacement of the floating member as changes in inductance between the sensor elements Z1 and Z2 by comparing the voltages at the displacement point B and the reference point A. However, the magnetic bearing and controller are connected by cables as described above having capacitances C1, C2, and the like. These capacitances C1 and C2 vary according to the length of the cables C. Accordingly, if the length of the cable changes after the sensor circuit is adjusted for the cable length, then the cable capacitances C1, C2, and the like will also change, as well as the voltage at the displacement point B between sensor elements Z1 and Z2 in relation to the reference point A of the bridge circuit. As a result, it is not possible to accurately detect the position of the floating member using this construction. Errors made in positional measurement will throw off the position of the floating member controlled according to this measurement. As a result, the sensor circuit must be adjusted whenever the length of cable is modified.
In view of the foregoing, it is an object of the present invention to provide a magnetic bearing controller capable of reliably controlling the position of a floating member at a target position, even when the length of the cable connecting the magnetic bearing and the controller is changed.
According to first aspect of the present invention, there is provided a magnetic bearing controller, which comprises an electromagnet for supporting a floating member in a floating state; sensors for sensing the floating position of the floating member; and a controller for supplying sensor signals and an exciting currents via cables to the sensors and electromagnets, respectively, in order to support the floating member at a predetermined floating position based on signals received from the sensors, wherein the controller comprises: a signal source for supplying an AC signal to the sensors; a pair of sensor drive circuits for supplying differential signals via cables connected to both ends of the serially connected sensor elements; two reference resistances that form a bridge circuit with the two sensor elements connected to the sensor drive circuit; and a sensor circuit for detecting a differential voltage between a center point of the sensor elements and a center point of the reference resistances connected by the bridge circuit. Whereby the cables are disposed with balanced condition and capacitance generated by the length of the cable is canceled.
With this construction, the AC signal source is connected to two amplifiers that drive the sensor elements Z1 and Z2 on the magnetic bearing side with balanced differential signals. As a result, the reference point A at a center point between the Ra and Rb serves as a virtual ground, while the displacement point B at a center point between the Z1 and Z2 also serves as a virtual ground in relation to the capacitance. Since the capacitance of the cables contrast with each other at points A and B, the effect of the cable capacitances are cancelled. Accordingly, if the lengths of the cables are changed, the capacitances C1 and C2 will change in proportion to the lengths, but the effects based on the differential output from the bridge circuit are canceled. Therefore, the differential voltage produced between points A and B does not change. As a result, the sensor circuit need not be adjusted even when the cable lengths are changed.
According to second aspect of the present invention, there is provided a magnetic bearing controller, which comprises an electromagnet for supporting a floating member in a floating state; a sensor for sensing the floating position of the floating member; and a controller for supplying a sensor signal and exciting current based on the signal received from the sensor via cables to the sensor and electromagnet, respectively, for supporting the floating member at a predetermined floating position, wherein the controller comprises: a signal source for supplying an AC signal to the sensor; a sensor drive circuit for supplying the signal from the signal source via cables connected to both ends of serially connected sensor elements; reference resistances having a center point; a sensor circuit for detecting a differential voltage between a center point of the two sensor elements connected in series and the center point of the reference resistances; a DC signal superimposing circuit for superimposing a DC signal on the AC signal output from the signal source; and a DC signal measuring circuit for detecting the DC signal on the controller side connected via the cables including the sensor elements, whereby the length of the cable is determined.
According to another aspect of the present invention, a voltage corresponding to the length of the cable is converted to a voltage corresponding to capacitance in the cable and added to the sensor circuit to compensate for changes in the length of the cable, whereby the floating position of the floating member does not change.
With this construction, a DC current is superimposed on the AC signal and the DC voltage component including the cable length and sensor elements is extracted by the DC resistance detecting circuit. The extracted DC voltage component is compared to a reference voltage in the DC resistance detecting circuit. The difference is output as a DC voltage corresponding to the length of the cable. Hence, the length of the cable can be determined by this DC voltage. By adding the capacitance of this DC voltage corresponding to the length of the cable to the sensor circuit as a compensating voltage, it is possible to compensate for the effects of capacitance caused by the length of the cable, thereby not changing the floating position of the floating member.
According to third aspect of the present invention, there is provided a turbo-molecular pump apparatus, which comprises a main pump section having an electromagnet for supporting a rotating member in a floating state and a sensor for detecting the floating position of the rotating member; and a controller for supplying a sensor signal and exciting current via a cable to the sensor and electromagnet respectively based on signals from the sensor in order to support the floating member at a predetermined floating position; wherein the main pump section and the controller are integrally formed as one unit and further comprising a cooling jacket through which cooling water flows for cooling both the main pump section and the controller.
With this construction, the main pump section and controller are formed integrally, thereby eliminating the need for a cable connecting the two, as well as the need for laying out the cable. This construction also eliminates the possibility of error in connecting the cables. By eliminating the cable, there is no need to adjust the controller based on the length of a cable. This configuration also introduces interchangeability between the main pump section and controller. Further the configuration decreases noises entering into the cable, and increases tolerance for noises.
Since both the main section and controller can be cooled simultaneously by the cooling water flowing through the cooling jacket, the controller does not require a cooling fan. When this apparatus is installed in a constant-temperature clean room or some other location that requires the room to be maintained at a constant temperature, using expensive clean air is not necessary for cooling the controller, thereby eliminating the source of a hotshot.